The introduction of foreign genes into a retroviral shuttle vector offers the opportunity of infecting a variety of human cell types at high frequency. The coding region of the v-Ha-ras oncogene was cloned into the murine retroviral shuttle vector pZip-neo SV(X). Plasmid DNA of bacterial clones containing the Ha-ras coding region was isolated and transfected into NIH 3T3 cells by the calcium phosphate precipitation method. One out of six clones transfected was biologically active in induction of transformed foci.